1. Field of the Invention
The present invention relates to an image reading apparatus and an image reading method.
2. Description of the Related Art
Conventionally, an image reading apparatus configured to read an original by so-called moving document reading has been known. In the moving document reading, a reading unit having a light source for irradiating light to an original is stopped at a moving document reading position. When an original conveyed by an automatic original conveyance apparatus (hereinafter, referred to as the ADF) passes through the moving document reading position, the reading unit reads the original by scanning it in a main scanning direction.
The reading unit stopped at the moving document reading position faces a lower surface of a moving document reading glass disposed at the moving document reading position so as to permit light irradiated from the reading unit to pass therethrough, and the original is conveyed by the ADF between an original guide and the moving document reading glass along an upper surface of the moving document reading glass.
In the moving document reading, if there is a contaminant such as dirt (including dust), stain, or flaw on the moving document reading glass, an image streak along a sub-scanning direction appears in an image of the read original due to the contaminant being read.
To obviate this, various methods for detecting a contaminant adhered to the moving document reading glass have been proposed.
For example, in a method described in Japanese Laid-open Patent Publication No. 2005-045462, an original guide is read in a state before an original is conveyed up to the reading position. Then, leading and trailing end portions of an original passing through the reading position are read, and portions of the original passing through the reading position, other than the leading and trailing end portions thereof, are also read. If pixels of plural consecutive reading lines which are the same in density are present at the same main scanning direction position in respective main scanning direction line data obtained by the readings, these pixels are detected as abnormal pixels. With this method, only dirt adhered to the moving document reading glass can be detected as abnormal pixels and can be corrected. Since the dirt detection is performed each time one original is read, dirt conveyed together with an original up to the moving document reading glass can also be detected.
However, in a case that abnormal pixels are detected by the method disclosed in Japanese Laid-open Patent Publication No. 2005-045462 based on a result of the reading of the original guide in a state before an original is conveyed up to the reading position and a result of the reading of original end portions, normal pixels are sometimes erroneously detected as abnormal pixels and unnecessary correction is performed, if the density of dirt adhered to the original guide is close to the density of a base surface of the original end portions or close to the density of an image of the original end portions. To correct abnormal pixels, there is a method for replacing abnormal pixels with adjacent pixels, for example. However, since a corrected portion becomes visually noticeable in a case, for example, that an original image is a screen image, it is preferable that unnecessary correction be avoided as much as possible.